Degenerative amplifier



OSCILLAT/ONS y 1941. B. w. v. INGEN SCHENAU I 2,250,606

.DEGENERATIVE AMPLIFIER TOQOURCEOF QCl/OLTA E INVENTOR.BERNARDUSWMJNGENSCl/ENAI/ ATTORNEY.

Patented July 29, 1941 Bern'ardus W. v.

Netherlands, assignor,

to Radio Corporation Ingen Schenau, Eindhoven, by mesne assignments,

America, New York,

N. Y., a corporation of Delaware Application July 28, 1939, Serial No.286,992 In Germany April 1, 1939 Claims.

The present invention relates to an amplifying system having negativefeedback which acts on the input circuit of an amplifying tubecomprising a directly heated cathode.

The known negative feedback, which is frequently utilized withindirectly heated amplifying tubes and with which the voltage fed back(and which voltage is derived from the output transformer of theamplifier) is supplied to a resistance in the cathode lead of theamplifying tube to the input circuit of which the negative feedbacktakes place, is unsuitable for systems comprising directly heatedamplifying tubes. On the one hand, the resistance in the cathode leadwould have to be of small value to avoid the heating current bringingabout an excessive drop of voltage across the said resistance, while, onthe other hand, the resistance would have to be large in order to avoidexcessive damping of the output transformer by which the negativefeedback voltage is supplied to the said resistance.

According to the present invention, this drawback is removed if thenegative feedback voltage is supplied to the directly heated cathode ofthe amplifying tube on the input circuit of which the negative feedbackis operative.

The invention will be more clearly understood by reference to theaccompanying drawing which represents, by way of example, an amplifiernegatively coupled back in this way.

The amplifier shown comprises two amplifying tubes 1 and 2 which arecoupled with one another by means of a resistance-condenser coupling 3,4, 5. The oscillations to be amplified are supplied to input terminals 6and 1, and the amplified oscillations are taken from the secondarywinding of an output transformer B. The amplifying tubes l and 2 havedirectly heated cathodes which are supplied by a source of directcurrent (D. C.) voltage connected between terminals 9 and Ill.

Negative feedback to the input circuit of the amplifying tube l isobtained by supplying a negative feedback voltage taken from thesecondary winding of the output transformer to the cathode of theamplifying tube I. To that end a portion l I of the secondary winding ofthe output transformer is connected in series with the cathode of theamplifying tube I so that an alternating voltage, dependent upon thevoltage induced in the portion ll of the secondary winding of the outputtransformer, is applied to the cathode of the amplifying tube I. Betweenthe grid and the cathode an alternating voltage is thus set up which isin counter-phase with the voltage to be amplified and which is suppliedby the terminals 6 and l to the grid, so that negative feedback isobtained.

The resistance of the filament of the amplifying tube is comparativelyhigh (i. e. of the order of magnitude of 30 ohms, and, therefore, bringsabout only slight damping of the portion ll of the secondary winding ofthe output transformer, which portion is connected in paralleltherewith, more particularly if only part of the secondary winding isincluded in the heating circuit of tube l, as is the case in the systemshown. Furthermore, in the usual output transformers the D. C.resistance of the secondary winding is small so that the heating currentof the tube I does not produce an appreciable voltage drop across thiswinding.

Impedances may be incorporated in the negative feedback circuit in orderto influence the frequency characteristic of the amplifier. Such animpedance, consisting of the parallel connection of an inductance coill2, a condenser I3 and a resistance I4, is represented in the drawing.This parallel connection is tuned in such manner that it constitutes ahigh impedance for high frequencies with the result that for highfrequencies the negative feedback decreases, and consequently theamplification increases for these frequencies.

What is claimed is:

1. In combination with a tube having at least a grid, cathode, andoutput electrode, means for impressing signals between the grid andcathode, means for directly heating the cathode, a second tube, providedwith input and output electrodes, having its input electrodes coupled tothe first tube output electrode, an output transformer coupled to theoutput electrodes of the second tube, and at least a portion of thetransformer secondary in circuit with said heating means.

2. In combination with a tube having at least a grid, cathode, andoutput electrode, means for impressing signals between the grid andcathode, means for directly heating the cathode, a second tube, providedwith input and output electrodes, having its input electrodes coupled tothe first tube output electrode, an output transformer coupled to theoutput electrodes of the second tube, and at least a portion of thetransformer secondary in circuit with said heating means arranged todegeneratively feed back signal voltage to said grid and cathode.

3. In combination with a tube having at least a grid, cathode, andoutput electrode, means for impressing signals between the grid andcathode,

4. In a signal amplifier network having a plurality of tubes coupled incascade, at least one.

tube having a cathode in direct circuit with a heating current source,an output circuit connected to a later one of the tubes, at least aportion of said output circuit being included solely in the said heatingcircuit whereby signals are degeneratively applied to said one tubeinput circuit.

5. In a signal amplifier network having a plurality of tubes coupled incascade, at least one tube havinga cathode in direct circuit with aheating current source, an output circuit connected to a later one ofthe tubes, at least a portion of said output circuit being includedsolely -'in the said heating circuit whereby signals are degenerativelyapplied to said one tube input circuit, said output circuit comprising atransformer, and said portion being a section of the transformersecondary.

BERNARDUS W. v. INGE'N SCHENAU.

